(1) Field of the Invention
The present invention relates to a driving device for an optical system, such as a scanner, and more particularly to a driving device for an optical system used for the document reading unit of a copying machine.
(2) Related Art
In a copying machine of stationary document scanning type, a document is placed on a platen glass, and an optical system provided below the platen glass is driven in parallel with the surface of the document by a predetermined driving device to optically read the image of the document.
When scanning a document, inconsistencies in the velocity of the scanner (scanning velocity) or in the timing at which scanning is commenced can lead to inconsistencies in the reproduction image, resulting in low image quality. For this reason, a pulse motor is employed as a driving source for the scanner driving device, and the generation of driving pulses for this pulse motor is controlled to obtain a stable scanning velocity and read starting position.
In this kind of conventional driving device, however, when the pulse motor is activated, the pulse motor is controlled to accelerate with constant acceleration until the scanner reaches a predetermined velocity (scanning velocity), at which point there have been problems regarding fluctuations in the velocity of the scanner. As shown in FIG. 1A, when the pulse motor accelerates from start with constant acceleration as shown by the solid line Q, the actual velocity at which the scanner is moving traces the curved line P drawn as a broken line. When the pulse motor reaches a predetermined velocity Vc, the scanner overshoots this velocity and thereafter moves with a fluctuating velocity. This overshoot is caused by a sudden release of tension that has built-up in the materials, such as wires, timing belt, and tension spring, forming the power transmission mechanism between the pulse motor and the scanner, with it being very difficult to eliminate the elasticity of these materials. The fluctuations in velocity caused by the overshoot adversely affect the image reading accuracy, resulting in a blurred reproduction image.
To avoid such problems, U.S. Pat. No. 4,586,808 discloses a method for preventing occurrences of overshoot and fluctuations in velocity by providing a deceleration region before the scanner reaches the predetermined velocity, as shown by a solid line R in FIG. 1B.
According to the prior art, fluctuations in velocity of the scanner can be prevented when the predetermined velocity is reached, although when a deceleration region is provided before the predetermined velocity is reached, a force due to the inertia of the moving scanner is transmitted from the scanner to the pulse motor via the power transmission mechanism. To endure the force due to the inertia of the scanner, the pulse motor needs to be large, or in other words have high torque, which increases the space required by the motor and leads to increases in the cost of the equipment.
The first object of the present invention is to provide a driving device for an optical system that can effectively prevent fluctuations in the velocity of the optical system without using a large-sized motor.
The second object of the present invention is to provide a method for driving a driving device that can effectively prevent fluctuations in the velocity of the optical system without using a large-sized motor.
The first object of the present invention can be achieved by providing an optical system driving device comprising a driving unit for driving an optical system, and a controller for controlling the driving unit to create an easement interval for temporarily easing acceleration during an acceleration period in which the optical system accelerates to reach a predetermined velocity. The first object of the present invention can be achieved by providing a driving device comprising a driving unit for driving a driven object, and a controller for controlling the driving unit to create an easement interval for temporarily easing acceleration during an acceleration period in which the driven object is driven at a constant acceleration rate or an increasing acceleration rate.
The second object of the present invention is to provide a method for operating a driving device, comprising the steps of: performing an acceleration operation to move a driving unit in a predetermined direction; performing an easement operation to ease acceleration; and terminating the easement operation to resume the acceleration operation.